Production of nuclear fuel

ABSTRACT

A process for the production of plutonium-containing mixed oxide nuclear fuel. The process comprises selecting a plutonium-containing solution, determining the plutonium isotopic composition of the solution, precipitating particulate plutonium oxide from the solution, forming a mixture comprising the plutonium oxide and a particulate uranium oxide, and forming nuclear fuel compacts from the mixture. The amount of plutonium in the mixture is controlled in accordance with a given equation.

BACKGROUND OF THE INVENTION

This invention related to the production of nuclear fuel, and moreparticularly to the production of plutonium-containing mixed oxidenuclear fuel (hereinafter referred to as "MOX fuel"), such as mixedplutonium oxide and uranium oxide nuclear fuel.

The reactivity of MOX fuel assemblies over their lifetime in a nuclearreactor can be affected by plutonium isotopic variations if no specialmeasures are taken to counter the effect of the variations. Plutoniumisotopic variations can also cause local hot spots in MOX fuelassemblies which can increase within-assembly peaking factors should thehighest rated fuel rod contain plutonium with a higher than specifiedfissile plutonium fraction. To alleviate these problems the currentpractice by MOX fuel manufacturers is either to homogenise all thePu(NO₃) ₄ required for an entire reactor reload of MOX fuel assembliesor, alternatively, to use a `mix and match ` approach of blendingPu(NO₃)₄ from various separate batches in order to meet the specifiedplutonium isotopic composition.

SUMMARY OF THE INVENTION

According to the present invention there is provided a process for theproduction of plutonium-containing mixed oxide nuclear fuel, the processcomprising selecting a plutonium-containing solution, determining theplutonium isotopic composition of the solution, precipitatingparticulate plutonium oxide from the solution, forming a mixturecomprising the plutonium oxide and a particulate uranium oxide, andforming nuclear fuel compacts from the mixture, the amount of plutoniumin the mixture being controlled in accordance with the equation:

    Δ c.sub.Pu /C.sub.Pu =α.sub.8 αf.sub.238 -α.sub.9 αf.sub.239 +α.sub.0 αf.sub.240 -α.sub.1 αf.sub.241 =α.sub.2 αf.sub.242 -[(1-C.sub.Pu)/C.sub.Pu ]α.sub.5 αf.sub.235 +α.sub.Am αf.sub.Am

The left side of the above equation gives the relative change in totalPu content needed to compensate for deviations in the Pu, U²³⁵ and Am²⁴¹relative to the specified composition. f₂₃₈, f₂₃₉, f₂₄₀, f₂₄₁ and f₂₄₂represent the absolute perturbations in the Pu²³⁸, Pu²³⁹, Pu²⁴⁰, Pu²⁴¹and Pu²⁴² isotopic fractions, respectively. f₂₃₅ is the absoluteperturbation for U²³⁵ and f_(Am) that for Am²⁴¹ The α's, which are allpositive in value, are related to the partial derivatives of lifetimeaveraged reactivity to variations in the individual isotopes. The α'svary from reactor to reactor depending on the fuel design and the fuelmanagement scheme. For pressurized water reactors, the coefficients havevalues in the region of α₈ ≈1.5, α₉ ≈2.0, α₀ ≈1.0, α₁ ≈2.0, α₂ ≈3.0,[(1-C_(Pu))/C_(Pu) ]α₅≈30.0 and α_(Am) ≈3.0. The signs of the terms inthe equation thus reflect whether the associated isotope contributes tofissions or is an absorber. The large numerical value for the U²³⁵coefficient merely reflects the fact that the bulk of the MOX fuelconsists of UO₂, and a given change in the concentration of U²³⁵corresponds to a large absolute change in the concentration of U²³⁵atoms. The plutonium-containing solution may comprise Pu(NO₃)₄, or someother solution such as a sulphate.

It is an advantage of the invention that variations in plutoniumisotopic composition can be compensated for within a fuel reloadquantity by varying the weight of plutonium, thus obviating the need fora complex and costly homogenisation stage. Thus use of the inventionshould ensure that the reactivity of a particular MOX fuel assemblyaveraged over its lifetime from initial insertion in a reactor to finaldischarge from the reactor substantially matches that of a specifiednuclear fuel assembly which might be a uranium dioxide fuel assemblyco-resident in the reactor or another MOX fuel assembly. Hence, theparticular MOX fuel assembly should contribute its full share to thereactivity of the reactor.

The plutonium oxide may be precipitated, mixed with the uranium oxide(eg UO₂), and formed into fuel pellets (eg cylindrical pellets) bymethods known in the art. It will be understood that as used herein theterm "uranium oxide" includes appropriate oxides such as uraniumdioxide.

We claim:
 1. A process for the production of plutonium-containing mixedoxide nuclear fuel, the process comprising selecting aplutonium-containing solution, determining the plutonium isotopiccomposition of the solution, precipitating particulate plutonium oxidefrom the solution, forming a mixture comprising the plutonium oxide anda particulate uranium oxide, and forming nuclear fuel compacts from themixture, the amount of plutonium in the mixture being controlled inaccordance with the equation:

    ΔC.sub.Pu /C.sub.Pu =α.sub.8 Δf.sub.238 -α.sub.9 Δf.sub.239 +α.sub.0 Δf.sub.240 -α.sub.1 Δf.sub.241 +α.sub.2 Δf.sub.242 -[( 1-C.sub.Pu)/c.sub.Pu ] α.sub.5 Δf.sub.235 +α.sub.Am Δf.sub.Am

f₂₃₈, f₂₃₉, f₂₄₀, f₂₄₁ and f₂₄₂ representing the absolute perturbationsin the Pu²³⁸, Pu²³⁹, Pu²⁴⁰, Pu²⁴¹ and Pu²⁴² isotopic fractions,respectively, f₂₃₅ being the absolute perturbation for U²³⁵, and f_(Am)that for Am²⁴¹, the α's, all being positive in value and being factorsrelated to the partial derivatives of lifetime averaged reactivity tovariations in the individual isotopes.
 2. A process as claimed in claim1, wherein for said nuclear fuel for a pressurized water reactor,

    α.sub.8 ≈1.5, α.sub.9 ≈2.0, α.sub.0 ≈1.0, α.sub.1 ≈2.0, α.sub.2 ≈3.0, [(1-C.sub.Pu)/C.sub.Pu ]α.sub.5 ≈30.0 and α.sub.Am ≈3.0


3. A process as claimed in claim 1 and wherein the uranium oxidecomprises uranium dioxide.
 4. A process as claimed in claim 1 andwherein the compacts comprise cylindrical pellets.